The invention relates to an operational amplifier (OA) or comparator circuit with minimized offset voltage and drift, particularly for analog and/or digital systems requiring high speed and high accuracy signal processing, such as integrators, analog-to-digital converters, etc.
An OA is an amplifier having a pair of inputs for amplifying an input signal applied thereto. In particular, an inherent parasitic offset voltage is present between inputs of any OA and depends on temperature, represented by an offset drift, the level of the input signal, represented by a common mode rejection ratio (CMRR), and the frequency of the amplified input signal. Therefore, an error voltage present between the inputs of the OA is a momentary value of the offset voltage.
A reduction of offset voltage and drift of an OA is ordinarily accomplished in a circuit incorporating high speed and precision OAs. An exemplary approach is described in U.S. Pat. No. 4,634,996 by the same inventor.
A combination of fast and accurate techniques in one system results mostly in a greatly decreased speed of fast components, having inherently a poor DC performance, due to a sluggish response of correcting components, introduction of parasitic capacitances, etc. The system structure must be optimized due to specific requirements of precision amplifiers. Other techniques demand so called autozero modes which degrade the system performance. Dynamically generated errors are not meaningfully reduced.
The invention is intended to provide OA and comparator circuits retaining high speed and having a drastically increased accuracy. No correcting amplifier is necessary. The gain set by external components is insignificant. An autozero mode is required only in case of a comparator since that is the only way to accurately obtain its error voltage. Widely available high speed switching components can be used. An ampler circuit according to the present invention includes an amplifier means having a pair of inputs, means for coupling an input signal to the input and including capacitor means coupled in series between the input signal and one of the inputs for storing an error voltage present between the inputs, and means for applying the error voltage across the capacitor means. Throughout the drawings, similar references denote similar parts.